As far as I remember from a friend who owned such a camera, unlike in other 135 cameras, in the Canon 1000 SLRs, when you put a 135 cartridge into it, and then when you close the back the whole roll of film will be spooled on the receiving spool. Then, each time after you shoot, the image just taken is rewound back into the cartridge. This way, if you accidentaly open the camera in the middle of the film, all pictures already taken are saved inside the cartridge.

Paul